1. The Field of the Invention
The present invention relates to the formation of a tungsten nitride film situated on a semiconductor wafer. More particularly, the present invention is directed to the formation of a large grain tungsten nitride film situated on a semiconductor wafer in a process suitable for forming a diffusion barrier and for forming a titanium salicide stack structure with a tungsten nitride cover layer.
2. The Relevant Technology
In the manufacturing of integrated circuits, barriers are often needed to prevent the diffusion of one material to an adjacent material. For instance, when aluminum contacts silicon surfaces, spiking can occur, and when aluminum comes into direct contact with tungsten, a highly resistive alloy is formed. Diffusion barriers are structures commonly used to prevent such undesirable reactions.
Titanium nitride has formerly been the material of choice for forming diffusion barriers and adhesive layers where conductive interfaces must be maintained. More recently, however, tungsten nitride has begun to appear more favorable and is in many applications replacing titanium nitride. Tungsten nitride has advantages over titanium nitride in that it has a lower resistivity and is thus more suitable for use in conductive interfaces in high speed applications. It is also more thermally stable, making it more suitable for the high temperature processing common in integrated circuit manufacturing.
One conventional method of forming tungsten nitride diffusion barriers is with chemical vapor deposition (CVD). Conventional chemical vapor deposition processes react tungsten with gaseous nitrogen at a high temperature in atmosphere of fluorine to form a film of tungsten nitride. Problems attendant to this process include the detrimental tendency of the fluorine to attack exposed surfaces of semiconductor wafers on which the diffusion barrier is being formed. The lack of cleanliness of chemical vapor deposition processes also presents problems. Consequently, the art has looked to other methods of depositing tungsten nitride films.
Physical vapor deposition (PVD) is another convention method of forming tungsten nitride diffusion barriers and is an alternative to the use of chemical vapor deposition for depositing tungsten nitride. The conventional physical vapor deposition technology involves reactive sputtering from a tungsten target in an atmosphere of gaseous nitrogen with an argon carrier gas. In this conventional saturated reactive sputtering mode, the volume ratio of nitrogen (N.sub.2) to the argon carrier gas is selected such that the tungsten target is fully nitrided by surface dissociated nitrogen.
This type of conventional PVD process is highly reactive and causes simultaneous high density, nonuniform nucleation and grain growth, and results in a highly columnar, small grain film with a high resistivity. The small grain size, when the grains come into contact with adjacent layers such as aluminum which is of a large grain size, tends to cause stress at the interface between the layers and can cause the layers to peel away from each other. Also, the high amount and irregularity of the grains formed by the conventional process tend to cause voids, which give rise to electromigration and consequently, reduced diffusion barrier abilities. Voids are especially prone to forming at interfaces between adjoining layers.
A further problem with the conventional physical vapor deposition process is a columnar structure that is exhibited by the resulting film. The columnar structure, which appears as spikes between the grains, serves as a channel for diffusion and reduce the effectiveness of the structure as a diffusion barrier.
One application for tungsten nitride films is the formation of diffusion barriers between the tungsten of tungsten plugs and adjoining metallization layers on the surface of the wafer. Such a diffusion barrier is shown in FIG. 1. Therein is shown a tungsten plug 14 extending down to a silicon substrate 10 with an overlying metallization layer 16 and an intervening diffusion barrier 12. The tungsten plug structure is one example of an application where tungsten nitride has been found as a suitable replacement for titanium nitride, as it is easily formed over the tungsten plug. Nevertheless, void formation and interfacial stress inherent to the conventional physical vapor deposition processes, along with the aforementioned problems associated with fluorine processing and cleanliness for chemical vapor deposition processes are detriments to the use of tungsten nitride for such applications.
A further application where an improved method for forming tungsten nitride films could be favorably used is in the formation of low resistivity tungsten nitride/titanium silicide stack. A titanium silicide (TiSi.sub.2) self aligned diffusion barrier, known as titanium salicide, is formed by sputtering titanium on a polysilicon and annealing the deposited titanium at 650.degree. C. after masking in a gaseous nitrogen environment to form titanium salicide in a C-49 phase. A second anneal at 850.degree. C. transforms the titanium salicide to a more thermally stable C-54 phase and is followed by a standard wet strip. Titanium salicide stacks are commonly used for forming word and bit lines in DRAM memory structures and for forming local interconnects to CMOS gate structures.
The problems exhibited by conventional titanium silicide structures include agglomeration at the titanium silicide and polysilicon interface and decomposition of the titanium silicide back into titanium and silicon at high temperatures that results in high resistivity.
It is apparent from the above discussion that a need exists for a new process of forming a high quality tungsten nitride film which overcomes the problems existing with conventional chemical vapor deposition and physical vapor deposition processes, and which can be used to form a suitable diffusion barrier that has low resistivity, large grain size, low interfacial stress, and which is thermally stable. It is also apparent that such a process would be highly beneficial if it were compatible with and solved the aforementioned problems existent with processes for forming the titanium salicide stack structure of the prior art.